1. Field of the Invention
The present invention relates to a crystal oscillator in which a crystal blank and an IC (integrated circuit) chip are accommodated in a single vessel and a method of fabricating the same. In more particularly, the present invention relates to a crystal oscillator in which the IC chip is mounted to the vessel through a bump by using ultrasonic thermocompression bonding and a method of fabricating the same.
2. Description of the Related Art
A crystal oscillator is widely utilized as a device for providing a reference frequency and a reference time in various electronic apparatus including communication equipment. Recently, as is represented by portable equipment such as a portable telephone, the size of the apparatus becomes smaller and smaller. Thus, the crystal oscillator is also requested to be made small. The size requested for the crystal oscillator is, for example, 3 mm.times.5 mm for the bottom face thereof and 1 mm for the height itself. For this reason, when an IC chip included in the crystal oscillator is mounted on a package or the like, the manner of mounting is changed from a conventional wire bonding to a face down bonding (FDB) in which one major surface of the IC chip having terminal electrodes formed is brought face down and bonded to an opposing substrate. As an example of the face down bonding, there has been proposed a bonding method using ultrasonic thermocompression bonding employing a bump.
FIG. 1 is a diagram showing a cross-sectional view of an arrangement of a general crystal oscillator. FIG. 2 is a magnified cross-sectional view of a part A of FIG. 1.
A crystal oscillator is a device in which IC chip 2 and crystal blank 3 are accommodated in vessel main body 1 made of laminated ceramic and cover 4 is bonded onto vessel main body 1 so as to encapsulate therein IC chip 2 and crystal blank 3. Cover 4 is made of ceramic and bonded to vessel main body 1 at the opening face thereof with glass 10, for example. Vessel main body 1 is formed to have a concave portion. A pair of crystal terminals (not shown) are provided on the upper side wall of the concave portion serving as an open end of the concave portion. Also, a circuit pattern connected to the crystal terminal is provided on the bottom surface of the concave portion. Vessel main body 1 has outer electrodes 5 formed on the external bottom and side surfaces of vessel main body 1 so that outer electrodes 5 are connectable to a circuit pattern through a laminated layer face.
Crystal blank 3 has an excitation electrode (not shown) on each of the major surfaces. Also, crystal blank 3 has a leading electrode (not shown) which extends from the excitation electrode toward the both ends of one edge of crystal blank 3. The leading electrode is electrically and mechanically connected to the above-described crystal terminal through conductive adhesive 9, whereby crystal blank 3 can be held within the vessel main body 1.
IC chip 2 is arranged as a flip chip in which electronic circuit components are integrated together with an oscillating circuit which uses crystal blank 3. The flip chip is a so-called bare chip as it has been cut off from a semiconductor wafer such as a silicon wafer having undergone processes of semiconductor device fabrication. In the present case, IC chip 2 has a conventional arrangement suitable for achieving electrical connection by using wire bonding. That is, as shown in FIG. 3, in order to facilitate the process of wire bonding, a plurality of terminal electrodes (IC terminals) 6 are provided along a pair of sides opposing to each other on one major surface of IC chip 2. An IC terminal for power supply and an IC terminal for ground are provided at corner portions of the major surface diagonally opposing to each other, in accordance with a standard.
Now, discussion is made on a case where IC chip 2 is mounted on a package or the like by means of wire bonding. When wire bonding is employed for mounting the IC chip, a thinned wire made of gold (Au) is employed as the bonding wire. One end of the wire is connected to each of IC terminals 6. In order to decrease the electric resistance caused by the wire, particularly in the case of connection with the power supply IC terminal, unit terminals 6a and 6b each having the same size as that of an IC terminal for signal transmission, are connected to each other. Also, when an IC terminal for ground is formed on the IC chip, a pair of unit terminals 6c and 6d are connected to each other. Thus, wires can be connected in parallel, with the result that the resistance caused by the wire can be decreased to half. IC terminals include a terminal for signal output, a terminal for implementing a standby function and a terminal for connection to an external terminal or the like as well as a terminal for power supply and a terminal for ground. In the example shown in FIG. 3, a couple of unit terminals for ground and two IC terminals for signal transmission are arrayed along one side 10a of the chip. Further, a couple of unit terminals for power supply and five IC terminals for signal transmission are arrayed along the other side 10b of the chip.
While the above description was made on a case where IC chip 3 shown in FIG. 3 is mounted by means of wire bonding, the following is a description made on a case where IC chip 2 can be mounted by means of face down bonding.
When the IC chip is mounted on the package or the like by means of face down bonding, bumps 7 each having a ball-shape made of gold (Au) or the like are formed on IC terminals 6 which are necessary to be electrically connected. In general, IC chip 2 is provided with a test terminal which is utilized when IC chip 2 is subjected to test before it is mounted. However, when IC chip 2 is subjected to test, the test terminal is directly connected to a test probe or the like. Further, when IC 2 is mounted on a package or the like, it is unnecessary for the test terminal to be electrically connected to a circuit on the package. Therefore, it is unnecessary for bump 7 to be provided on the test terminal. As for IC terminals for power supply, only one of unit terminals 6a and 6b has bump 7 formed thereon. Similarly, as for IC terminals for ground, only one of unit terminals 6c and 6d has bump 7 formed thereon. This is because the manner of mounting is not wire bonding and hence there is no resistance caused by a wire. Thus, it is expected that desired electrical connection will be accomplished by forming a bump on only one of the two unit terminals connected to each other. Each of bumps 7 is brought contact to terminal electrode (circuit electrode) 8 formed at the tip end of the circuit pattern configured on the bottom surface of concave portion 1a of vessel main body 1. Thus, IC chip 2 is fixed to vessel main body 1 by means of face down bonding. In the example shown in FIG. 3, three bumps 7 in total are provided along side 10a of the major surface of the IC chip while five bumps 7 in total are provided along side 10b of the major surface of the IC chip.
Upon face down bonding of IC chip 2, of major surfaces of IC chip 2, a major surface on which a circuit, IC terminals 6 or the like are formed is brought into an opposing relationship with the bottom surface of concave portion 1a of vessel main body 1. Then, IC chip 2 and vessel main body 1 are registered with each other so that bumps 7 and circuit terminals 8 of the circuit pattern are aligned with each other. When each of bumps 7 and corresponding one of circuit terminals 8 of the circuit pattern abut against each other, ultrasonic thermocompression, for example, is effected to make bumps 7 and circuit terminals 8 be fixed to each other. Ultrasonic thermocompression is effected in such a manner that a nozzle (not shown) is applied to the center of the upper surface of the IC chip and ultrasonic wave is supplied to the IC chip while pressing the same. In this way, bumps 7 are rubbed with circuit terminals 8 while being crashed into an elliptical shape. Thus, bumps 7 are bonded to circuit terminals 8. In this case, solid phase diffusion is effected in the metal (in this case, gold) and bonding is accomplished.
If IC chip 2 is designed from the initial stage of designing without intention that the IC chip undergoes wire bonding but with intention that the IC chip undergoes face down bonding, to provide a couple of unit terminals for the IC terminals for power supply and ground is not considered as a significant matter. In other words, it is considered that a single unit terminal is sufficient for each of the IC terminals for power supply and ground, similarly to IC terminals 6 for signal transmission use. FIG. 4 shows an arrangement of an IC chip in which only one unit terminal is provided for each of the terminals for power supply and ground. FIG. 5 shows an example of a circuit pattern which is matched with IC chip 2 shown in FIG. 4 when it is provided on bottom surface 1a of vessel main body 1. Circuit pattern (conductive run) 9 is formed in such a manner that a first layer as a base electrode layer made of tungsten (W) is printed and burned, and thereafter a layer made of gold (Au) is formed by electrolytic plating on the surface of the tungsten layer. In an ordinary manner, circuit terminal portion 8 shown in FIG. 5 with half tone notation, which serves as an end contact area of the conductive run, is exposed, and portions other than the end contact area are covered with insulation material (not shown) such as alumina.
If face down bonding is employed, it becomes possible to make small the projection area of the crystal oscillator as compared with when wire bonding is employed. Further, the height of the crystal oscillator upon being mounted on a printed circuit board or the like can be decreased, which fact will contribute to high density mounting when the crystal oscillator is utilized in equipment or the like.
However, in the crystal oscillator of the above arrangement, connection between IC chip 2 and circuit terminal 8 tends to become unsatisfactory, with the result that the IC chip becomes useless though the IC chip is expensive. As a consequence, yield and productivity of the crystal oscillator are lowered.